The present invention relates to electrochemichromic solutions and devices based thereon. Such solutions are well-known and are designed to either color or clear, depending on desired application, under the influence of applied voltage.
Such devices have been suggested for use as rearview mirrors in automobiles such that in night driving conditions, application of a voltage would darken a solution contained in a cell incorporated into the mirror (U.S. Pat. No. 3,280,701, Oct. 25, 1966). Similarly, it has been suggested that windows incorporating such cells could be darkened to block out sunlight, and then allowed to lighten again at night. Electrochemichromic cells have been used as display devices and have been suggested for use as antidazzle and fog-penetrating devices in conjunction with motor vehicle headlamps (British Patent Specification 328017, May 15, 1930).
U.S. Pat. No. 4,090,782 to Bredfeldt et al., U.S. Pat. No. 4,752,119 to Ueno et al. (June, 1988), Chemical Abstract 86:196871c, 72-Electro. Chemistry, Vol. 86, 1977, I. V. Shelepin et al. in Electrokhimya, 13(3), 404-408 (March, 1977), O. A. Ushakov et al., Electrokhimya, 14(2), 319-322 (February, 1978), U.S.S.R. Patent 566863 to Shelepin (August, 1977), U.S. Pat. No. 3,451,741 to Manos, European Patent Publication 240,226 published Oct. 7, 1987 to Byker, U.S. Pat. No. 3,806,229 to Schoot et al., U.S. Pat. No. 4,093,358 to Shattuck et al., European Patent Publication 0012419 published Jun. 25, 1980 to Shattuck and U.S. Pat. No. 4,139,276 to Clecak et al. all disclose electrochemichromic solutions of anodic and cathodic electrochromically coloring components which provide self-erasing, high color contrast, single compartment cells. Such anodic and cathodic coloring components comprise redox couples selected to exhibit the following reaction: ##STR1## The redox couple is selected such that the equilibrium position of the mixture thereof lies completely to the left of the equation. At rest potential, the anodically coloring reductant species RED.sub.1, and the cathodically coloring oxidant species OX.sub.2 are colorless. To cause a color change, voltage is applied and the normally colorless RED.sub.1 is anodically oxidized to its colored antipode OX.sub.1, while, simultaneously, OX.sub.2 is cathodically reduced to its colored antipode, RED.sub.2. These cathodic/anodic reactions occur preferentially at the electrodes which, in practical devices, are typically transparent conductive electrodes. Within the bulk of the solution, the redox potentials are such that when RED.sub.2 and OX.sub.1 come together, they revert to their lower energy form.
This means the applied potential need only suffice to drive the above reaction to the right. On removing the potential, the system reverts to its low energy state and the cell spontaneously self-erases.
Such redox pairs are placed in solution in an inert solvent. Typically, an electrolyte is also added. This solution is then placed into a relatively thin cell, between two conductive surfaces. In most applications, at least one of the conductive surfaces comprises a very thin layer of a transparent conductor such as indium tin oxide (ITO), doped tin oxide or doped zinc oxide deposited on a glass substrate so that the cell is transparent from at least one side. If the device is to be used in a mirror, the second surface is typically defined by a relatively thin layer of transparent conductor such as indium tin oxide, doped tin oxide or doped zinc oxide deposited on another glass substrate, which is silvered or aluminized or otherwise reflector coated on its opposite side. In the case of solar control windows, the second glass substrate would of course not be silvered on its opposite side so that when the redox pair is colorless, the window would be entirely transparent.
A wide variety of cathodically coloring species, anodically coloring species, inert current carrying electrolytes and solvent systems are described in prior art. One problem with the prior art solvent-electrolyte-redox chemical systems in electrochemichromic devices relates to leakage current. When the electrochemichromic cell is colored by the application of voltage, the colored species OX.sub.1 and RED.sub.2 continually want to recombine and return to their equilibrium, colorless condition. The rate of recombination of the colored species OX.sub.1 and RED.sub.2 within the bulk of the solution is inversely proportional to their diffusion coefficient in the solvent used. In order to counteract the tendency of the colored species to recombine and go to the colorless equilibrium state, current must continually leak into the bulk solution to compensate for recombination.
Because current must flow across the conductive surface of the transparent conductor used on at least one of the substrates that sandwich the electrochemichromic cell, and because these transparent conductors have finite sheet resistance, applied potential will be highest adjacent to the bus bar connector typically located at an edge perimeter and will be lowest near the center of the device as current passes across the conductive glass surface to color remote regions. Thus, if the leakage current is high and/or the sheet resistance of the transparent conductor is high, the potential drop that ensues across the transparent conductor itself results in a lower potential being applied to remote regions. Coloration is therefore nonuniform with the edge regions nearest the bus bar coloring deepest and the central regions coloring lightest. Such nonuniformity in coloration is commercially undesirable. For a given transparent conductor sheet resistance, the lower the leakage current the more uniform the coloration. This is an important advantage; otherwise, a thicker and hence more costly and less transparent conductive coating would be needed to reduce the sheet resistance to accommodate the higher leakage currents seen with solvents suggested in the prior art.
Yet another disadvantage of higher leakage currents is their imposition of a drain on battery-power sources in some instances. If an electrochemichromic device were used in a sunroof, for example, it would be desirable to have the sunroof colored dark while the car is parked in a parking lot. If the current leakage is too great, the operator could find that the car battery has been drained as a result of current being drawn by the colored sunroof.
The addition of thickeners such as polymethylmethacrylate (PMMA) to the solvent will reduce leakage current. It will also reduce "segregation." When first bleached after being held for a prolonged period in the colored state, bands of color are seen adjacent to the bus bar connectors to the transparent conductive electrodes that sandwich the electrochemichromic solution. One problem with adding thickeners is that the solution can become so viscous that vacuum backfilling a thin electrochemichromic cell becomes commercially unfeasible.
As a result of these drawbacks, electrochemichromic solutions and devices based thereon have not achieved the degree of commercial success which they potentially could achieve.